How do I add a line break in display math mode?

Question

For instance, how do I add a break after the first line in

[
    a + b = c // what should I add here?
    a = c - b
]

Peter Grill · Accepted Answer

The [...] is used to typeset a single equation, not multiple equations.
You need to use an environment that allows for multiple equations. One way is to use gather, but I normally use align, both from the amsmath package:

Notes:

With display math environments if you leave blank lines in between you get additional vertical spacing and the possibility of a page break between a paragraph and following display.
If you don't want the equations numbered use the starred versions of the environments: gather* and align*.
A really good reference is Herbert Voss's  comprehensive review of mathematics in (La)TeX.

Code:
documentclass{article}
usepackage{amsmath}
begin{document}
begin{gather}
    a + b = c  
    a = c - b
end{gather}
%
With the align environment you can align equations:
%
begin{align}
    a + b &= c         
        a &= c - b
end{align}
end{document}

L.C · Answer

In case gather is not an option, for example, an imported command where everything is in math mode, consider make them a single-columned matrix without showing the symbol, then you can use .
Code:

[
begin{array}{c}
a + b = c   % just use 
a = c - b
end{array}
]

John D · Answer

Why not use two modes.
[
    a + b = c
]
[
    a = c - b
]

How do I add a line break in display math mode?

3 Answers

Notes:

Code:

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